1. Field of the Invention
The present invention relates to a lens material, an optical electronic component and an optical electronic device, and in particular, to a lens material having a reduced amount of birefringence, and optical electronic component including the same, and an optical electronic device including the same.
2. Description of the Related Art
As optical materials for an optical system, glass, plastics, synthesized quartz, calcium fluoride and other suitable materials are known.
Glass and plastics have low refractive indexes. For instance, a glass lens has a refractive index of substantially 1.5 (see, for example, JP No. 2859621 (Patent Literature 1)). When lenses having the same focal distance are manufactured using the glass, a curvature radius of the lens must be decreased. That is, when the glass is used, the thickness of the lens increases. Accordingly, miniaturization and thinning of the lens are difficult.
Furthermore, optical glasses made of materials having refractive indexes of substantially 1.7 to 2.0 have been developed. However, there is a problem in that as the refractive index increases, the coloring of the lens increases and the light transmittance in a short wavelength region (corresponding to a wavelength from blue to green) in the visible light region deteriorates.
On the other hand, with plastic lenses, a complicated shape can be inexpensively and easily molded. However, there is a problem in that, since the volume of the lens greatly varies as a result of environmental variations, such as temperature and humidity, the refractive index tends to vary, which causes variations in the focal distance (see, for example, S. Nagata, ZUKAI RENDU GA WAKARU HON, pp 56-59, (Jan. 20, 2003), First edition, Third printing (NIPPON JITSUGYOU SHUPPAN SHA) (Non-patent Literature 1)).
In addition, as an optical material for optical elements for wavelength conversion, optical diffraction, phase conjugate mirror and other properties, single crystals, such as lithium niobate and lithium tantalate, are known. The optical materials, which have refractive indexes of at least 2.0, can be miniaturized and thinned. However, the single crystals, being a uniaxial crystal, have different refractive indexes for ordinary light and extraordinary light. Accordingly, there is a problem in that birefringence is caused and doubling results. As a result, the single crystals could not be used in a lens and optical system.
There is a proposal for optical pickup lenses for magnetic optical disks, DVDs (Digital Versatile Disk) and other devices, in which lithium tantalate that generates the birefringence is used (see, for example, JP-A No. 11-312331 (Patent Literature 2)). However, a light incidence axis (light incidence direction) has to be set at an angle of at least 0° with respect to a crystal optical axis of a single crystal, (in particular, a crystal optical axis substantially coincides with a light incidence axis (within ±1°) or substantially 45° (within ±1°)). Furthermore, laser light that generates only a very mono-dispersed wavelength must be used and a target axis of the lens and an optical axis of the crystal must be precisely coincided. Accordingly, when, like in a general image pickup device, natural light (aggregate of lights having various wavelengths) comes in from all directions (angles), this proposal cannot be applied.
Lithium tantalate is a material that has a refractive index of at least 2.0 and high light transmittance in the visible light region. However, since the birefringence thereof is substantially 0.006, when light comes in from various directions, images are duplicated. Accordingly, it has not been used as a lens and optical material.